This event is used by the MegaZine class to signal scaling
changes of the page container (thus the display size of the pages).

This event is fired with a small delay after the last actual scaling change,
and will only be fired once for any major scaling change. For example, if the
user starts changing the window size and the page container adjusts
accordingly, if the user continually changes the size over a longer period of
time, no event will be fired during that time. Only when she stops dragging
will the event be fired, with the final scaling used.

Used by the MegaZine class when its page scaling changes, fired after a
small delay to make it easier to handle final scaling changes (i.e. when
the user zooms multiple steps don't fire for each step, just the last).